210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
We are thus led to regard the hook hairs of the crab as homologues 
of the otolith hairs of Macrura, and for these five reasons :— (1) The 
similarity in their structure. (2) Their similarity in position at the 
posterior end of the sac. (3) Otoliths are in contact with the hook 
hairs in larval stages, though not in the adult. (4) When the otoliths 
disappear, the development of the hook hairs is arrested. (5) Gland 
pores open through the chitin of their cushion, as they do through that 
of the crayfish and lobster, although they are not found in the other 
sensory regions of the sac. 
(2) The thread hairs are the largest, the most highly differentiated, 
and probably the most active sensory bristles of the otocyst. There 
are about thirty of them, arranged upon the large anterior sensory 
cushion in a regular row (Fig. A, set. fil.). These hairs are extremely 
attenuate. Measuring only two or three uw at the base, the straight or 
slightly bending shaft averages 320 in length; it is unfringed save 
at the very tip, where for a short distance it bears two rows of ex- 
tremely delicate pinnules. A peculiarity of this fringed tip is that 
it is not a coutinuation of the main shaft of the hair, but seemingly 
a diminutive hair in itself, sprouting from the latter. It makes a 
slight angle with the main shaft, the end of which projects a short 
distance beyond the base of the offshoot (Plate 10, Figs. 53, 54). 
The shafts of these hairs are directed out laterally, and slightly pos- 
teriorly, into the fluid contents of the sac, and they are so delicately 
attached at their bases that the slightest jar imparted to the liquid 
in which they float is sufficient to set them swaying. In alcoholic 
material they break off very easily. The shaft decreases somewhat in 
diameter towards its base and then suddenly enlarges. This enlarge- 
ment is attached to the floor of a deep cup-like socket, the orifice of 
which is large enough to give ample play to the shaft in its movements 
(Fig. 53). 
Straight attenuate hairs are found in the otocyst of the Megalops 
larva having the same relative position in the sac as the thread hairs 
of the adult. These hairs are aot in contact with otoliths, but each 
shaft is fringed with filaments throughout its whole length. They 
become differentiated in later stages into the peculiarly modified thread 
hairs. Hairs similar to those of the Megalops larva just described 
are also found in the otocyst of the adult lobster, situated on the 
median wall of the sac and projecting free into its lumen. They are 
similar in both larva and adult, and are probably in function accessory 
to the otolith hairs. They may be homologues of the thread hairs, 
